Add support for idle inhibition on Wayland

This uses the idle-inhibit protocol from wayland-protocols, to attach an inhibitor to the GdkSurface. It can’t do anything if the GtkWindow isn’t passed, which might warrant some documentation change to encourage users to not use NULL for this argument.

I’m not very familiar with the internals of GTK, so let me know if it would be better architectured some other way, I mostly copied over the Quartz variant and adapted it to Wayland.

This has been tested on Sway using testlogout.

Fixes #2202 (closed).